A number of reversibly variable light transmission technologies based on changes between light scattering and less or non-light scattering conditions have been proposed or commercialized for a variety of intended uses. These technologies generally involve electrically induced and/or thermally induced changes. The materials with thermally induced, reversible changes between light scattering and less or non light scattering states can be termed thermally reversible light scattering, (TRLS), materials.
Polymer dispersed liquid crystal, (PDLC), technologies generally have involved liquid crystal droplets either physically dispersed in a polymer matrix, see for example U.S. Pat. Nos. 5,089,904; 5,082,351; 4,838,660; 4,815,826 and 4,435,047 or liquid crystal droplets formed by phase separation during curing of a reactive monomer/liquid crystal solution and/or by solvent removal, see for example U.S. Pat. Nos. 5,530,566; 5,087,387; 5,021,188; 4,888,126; 4,688,900; 4,685,771; 4,673,255 and 4,671,618. Droplet formation in the later case is due to decreased solubility of the liquid crystal material in the polymer being formed as compared to a higher solubility of the liquid crystal material in the monomer prior to curing. Droplet formation may also be due the polymer and the liquid crystal both being soluble in a solvent whereas the liquid crystal is insoluble or immiscible in the polymer and forms droplets within the polymer as the solvent is removed.
In general with PDLC materials, the index of refraction of the liquid crystal droplets is different from the polymer matrix material and a layer of the droplet containing material is light scattering and thus appears translucent, frosted or white. When a layer of PDLC is provided between two transparent electrode layers, a voltage can be applied to change the index of refraction of the liquid crystal droplets. As the index of refraction of the droplets approaches that of the polymer matrix the PDLC layer decreases in light scattering and with a high enough applied voltage the PDLC materials can become quite clear. Thus these devices are electrically operated or electrooptic variable light scattering devices although thermally induced changes from light scattering to clear or TRLS changes have been described for these material in U.S. Pat. Nos. 5,087,387; 5,021,188 and 4,888,126.
A more recently proposed electrooptic variable light scattering technology is based on what is called polymer stabilized cholesteric texture, (PSCT). With this technology a cholesteric liquid crystal is mixed with a small amount of a reactive monomer, placed in a very thin film between conducting layers and the monomer is allowed to react while an applied electric field holds the liquid crystal material in a clear or low light scattering state known as the homeotropic texture. Thus, the small amount of polymer matrix formed during the curing process favors or stabilizes this texture to some extent and the liquid crystal returns to it in the future when voltages of adequate strength are applied across the liquid crystal layer. In the absence of an applied voltage the liquid crystal material goes to a light scattering, focal conic texture. Devices with this technology rapidly switch between light scattering with no applied voltage to fairly low light scattering with an applied voltage. Examples of these materials and devices are given in U.S. Pat. Nos. 5,847,798; 5,695,682; 5,691,795; 5,644,330 and 5,570,216. Reverse mode device are also possible in which there is relatively little light scattering in the no voltage applied condition and the device becomes light scattering when a voltage is applied. PSCT technology lends itself to TRLS as heating a PSCT material from its mesomorphic phase with focal conic texture to it isotropic phase causes the polymer stabilized material to change from light scattering to less light scattering or clear.
One type of variable light scattering technology, that depends exclusively on temperature changes to cause changes in the light scattering nature of the materials, is based on the phenomenon know as lower critical solution temperature, (LCST), see for example U.S. Pat. Nos. 5,615,040; 5,525,430; 5,404,245; 4,952,035; 4,877,675; 4,832,466; 4,816,518; 4,772,506; and 4,260,225. With this technology, as the temperature is raised, a clear, transparent solution of a polymer in a solvent reaches a critical point at a particular temperature at which the polymer comes out of solution or phase separates to form a highly scattering material. Layers of this type of material have been proposed for use in window or roof situations where increases in ambient, outdoor temperature are enough to cause the transition from clear to frosted. The amount of backward scattered light is significant enough to have such windows and roof elements considered for providing energy savings in buildings that use this technology. They have also been described for use in displays and thermal recording materials in U.S. Pat. Nos. 4,952,035; 4,832,466 and 4,734,359. A reverse mode system which is reported to have materials with upper critical solution temperature, (UCST), behavior is described in U.S. Pat. No. 4,900,135. Both the materials based on LCST and those based on UCST qualify as TRLS materials.
Another type of thermally controlled, variable light scattering technology is suggested for use as thermally reversible recording media. Generally a low melting, relatively low molecular weight material is contained in a polymer matrix in these TRLS materials. In most cases, at low temperatures this composite material is light scattering. As the temperature is raised, above a certain point the material turns clear. To be useful as a recording media, this clear state is maintained by cooling the cleared material immediately after the temperature for clearing is reached. To erase a recorded image, the material is heated to a temperature significantly above the clearing temperature and cooling from this temperature regenerates the light scattering state of the material. Thus the preferred materials are bistable since they are stable in either the light scattering or the clear state at normal room temperature depending on their thermal history. For examples of this technology, see U.S. Pat. Nos. 5,965,484; 5,780,387; 5,747,413; 5,700,746; 5,298,476; 5,278,128; 4,917,948 and 4,695,528. Similar bistable, TRLS behavior has been described for liquid crystal polymers in U.S. Pat. Nos. 5,589,237 and 5,426,009 and for liquid crystals contained in an inorganic polymer matrix formed from fumed silica, see U.S. Pat. No. 5,729,320.
Thermally reversible light attenuating materials for various display and window applications are described in U.S. Pat. No. 4,268,413 to Dabisch. The TRLS materials of this invention are reported to comprise a polymeric or resinous matrix material and at least one organic substance embedded in the matrix material as a discrete phase. The organic substances embedded in the matrix material are relatively low molecular weight materials with melting points near or slightly above the desired transition between states with more and with less xe2x80x9clight-absorbancexe2x80x9d. The change in the way the material attenuates light involves a change of the dispersed, embedded organic substance from a solid which is at least partially insoluble in the matrix material to a liquid which has an index of refraction that matches the index of refraction of the matrix. Alternatively, the change in light attenuation involves a change in the dispersed, embedded material from solid particles which have an index of refraction that matches that of the matrix to liquid droplets which are not soluble in the matrix and have an index of refraction that differs from that of the matrix.
Descriptions of electrooptic devices that are suggested for changes between clear and light scattering states are given by Beni, et al xe2x80x9cAnisotropic Suspension Displayxe2x80x9d Appl. Phys. Lett. 39(3), 195-197 (1981) and xe2x80x9cElectro-wetting Displaysxe2x80x9d Appl. Phys. Lett. 38(4), 207-209 (1981).
No description has been found of a polymer which undergoes a melting process from a solid to a liquid, said polymer being intimately interspersed in a second polymer for the purpose of providing a TRLS material. The TRLS materials of the present invention are termed thermoscattering, (TS), materials.
One manifestation of the invention is a low melting, polymeric material dispersed in a polymer matrix which together show TRLS properties.
Another manifestation of the invention is a first polymer which upon heating from a temperature below its melting point to a temperature above its melting point changes from a light scattering solid to a liquid, said first polymer being dispersed in a matrix provided by a second polymer to form a light scattering polymer material when cold and a less or non-light scattering material when hot.
The invention provides unique TRLS materials which go from light scattering at lower temperatures to less or non-light scattering at higher temperatures.
In one embodiment the invention is a material comprising an aliphatic polyester, a poly(olefin glycol) and/or a poly(olefin carbonate) interspersed in another polymer which shows TRLS properties.
In another embodiment the invention is a material comprising an aliphatic polyester, a poly(olefin glycol) and/or a poly(olefin carbonate) interspersed in polyurethane matrix which shows TRLS properties.
The invention provides a window which can reversibly change from a privacy condition to a non-privacy or clear condition.
The invention also provides a window which can reversibly change between displaying images or objects to obscuring images or objects.
The invention provides a layer which can reversibly change between being clear and frosted by cycling the temperature of the layer.
The invention also provides a reversible display.
The invention still also provides a temperature indicating material or device.
Another embodiment of the invention is a TRLS material which is part of a device such as a display or a window with an advantageous configuration and a control system which provides for low energy requirement for switching the material to and maintaining the material in its slightly or non-light scattering condition.
The invention uses materials which change from light scattering at a lower temperature to substantially less light scattering or clear at a higher temperature. The materials of the invention comprise a first polymeric material which upon heating from a temperature below its melting point to a temperature above its melting point changes from a solid to a liquid, this first polymeric material being interspersed in a second polymeric material which second polymeric material is a solid at temperatures below the melting point of the first polymeric material and remains a solid, at least for some range of temperatures, above the melting point of the first polymeric material. The second polymeric material serves as a matrix for the first polymeric material and is preferably, but not necessarily, a polymeric material which is crosslinked. The material formed by the interspersion of the first polymeric material in the second polymeric material preferably remains solid throughout the useful temperature range of the material and through out the transition between the light scattering state and the substantially less light scattering or clear state. It is also preferable that the liquid form of the first polymer be miscible with the polymer matrix formed by the second polymer. The materials of the invention may additionally comprise additives such as other polymers, ultraviolet light absorbers, visible light absorbers, infrared light absorbers, ultraviolet light stabilizers, visible light stabilizers, antioxidants, antiozonants, singlet oxygen quenchers, thermal stabilizers, plasticizers, solvents, dyes and/or pigments.
The invention provides windows which comprise the materials of the invention, such that windows of the invention change from light scattering to substantially less light scattering or clear as the temperature of the windows increases. The windows of the invention include multipane windows with the material of the invention comprised in one or more of the panes of the window. The windows, optionally comprise one or more heat reflective layers and/or one or more low emissivity layers.
The invention can provide devices which comprise the materials of the invention in association with a source of heat. The devices of the invention change from light scattering to substantially less light scattering or to a clear condition as the temperature of the devices is increased by the source of heat. The devices of the invention may comprise the source of heat and/or the means of bringing the source of heat into association with the materials of the invention.
The source of heat may be an electric power source which resistively heats a transparent or non-transparent electronically conductive material or layer in thermal contact or in thermal association with the devices of the invention. Examples are conventional strip heaters, wire heating coils, a thin metal foil or a metal film on a substrate and transparent heaters like a layer of tin doped indium oxide, (ITO), or fluorine doped tin oxide on a flexible or a rigid substrate. Other resistive heaters, that allow for light transmission, can be associated with or even embedded in the TS material and include an array of fine wires, a fine wire mesh and a metal grid pattern formed by additive or subtractive processes.
The source of heat may be a source of electromagnetic radiation such as ultraviolet, visible, infrared or microwave radiation in radiative contact with the devices of the invention. Examples are the sun, a light bulb, a microwave oven, a fire and a laser.
The source of heat may be an exothermic chemical reaction, such as the burning of a fuel in air, in thermal or radiative contact with the devices of the invention. Examples are a gas burning oven or grill, a gas torch and a fire like the accidental burning of a building or the intentional burning of wood in a fireplace.
The devices of the invention include variable transmission windows, privacy glass and panels, oven door windows, fireplace windows/doors, wood burning stove window, artistic and information displays, security glazing, variable reflectance mirrors, covers for light bulb fixtures, warning signs and temperature indicators. These and a variety of other uses are described in detail below.
Polymers are often characterized by various thermal transitions that take place as the temperature of the polymer is changed. Two of the most important transitions are glass transitions and solid to melt transitions, (see for example Principles of polymerization 3rd Edition, by G. Odian, John Wiley and Sons, (1991) and Textbook of Polymer Science, 3rd Edition, by F. W. Billmeyer Jr., John Wiley and Sons, (1984)). In some cases passing through the given Tm or m.p. for a polymer results in little perceived change in the viscosity or light scattering character of the polymer. However some polymers undergo a dramatic change from what appears to be a light scattering, possibly crystalline solid to a relatively low viscosity liquid at or near their given melting point. Examples of polymers with a change from solid to liquid behavior, at temperatures of interest for the materials and devices of the present invention, can be found with aliphatic polyesters, poly(olefin glycols) and poly(olefin carbonates). Preferably, the polymers for melting from a light scattering solid state to non-light scattering liquids are relatively low molecular weight polymers, (number average molecular weight of about 600 to 50,000), which form moderately to highly crystalline solids. However, the principal requirement is that the polymer change from a solid state which is capable of scattering light to a liquid state which has little or no light scattering behavior.
If a first polymer with this dramatic type of melting behavior is thoroughly interspersed in a properly chosen second polymer which does not exhibit such melting behavior, (over the temperature range of intended use of the polymer combination formed by the interspersion), it has been discovered that a material can be formed which is light scattering at low temperatures and which is significantly less light scattering or clear, (little or no light scattering), at higher temperatures.
The combination of polymers will be light scattering at low temperatures if the second polymer can provide an environment that allows the first polymer to assume its solid, light scattering form when the combination is cooled. The polymer combination will be substantially less light scattering or clear, when the temperature is raised, if the first polymer changes to a clear liquid-like state and the interspersion of the liquid-like first polymer in the second polymer is such that there are few if any regions of either polymer present which are large enough to scatter light due the size of the region and the difference in index of refraction between the first polymer and the second polymer. In the high temperature condition, the combination should be particularly clear or low in light scattering if the interspersed, liquid form of the first polymer is miscible with the matrix formed by the second polymer. The material of the invention are a particular class of the TRLS which we term thermoscattering, (TS), materials.
The first polymer should be chosen based on a significant change from a solid, light scattering state to a clear, liquid state on melting, for its ability to intersperse in a second or matrix polymer, for its ability to form a light scattering condition in the combination of the polymers at lower temperatures and for compatibility at higher temperatures between the liquid or liquid like state of the first polymer and the second or matrix polymer. Compatibility between the first and second polymers means that the combination of the two is clear or significantly lower in light scattering at temperatures above the melting point of the first polymer and preferable means that the two polymers are miscible at temperatures above the melting point of the first polymer. The first polymers useful in the invention are not liquid crystalline materials. The first polymers useful in the invention are ones which melt from a light scattering solid to a clear liquid in the temperature range of the intended use of the TS material and/or device containing the TS material. Preferably the first polymer has a moderate to highly crystalline solid state.
Preferably the first polymer is one or more aliphatic polyester, one or more poly(olefin glycol), or one or more poly(olefin carbonate). The first polymer may be a copolymer of two or more of these polymers. The first polymer may be a combination, mixture or blend of two or more or these polymers or their coploymers.
The term aliphatic polyester is meant to include simple polyesters polymers with the following polymeric structures in which n and m are independently chosen from any value from 1 to 10:
xe2x80x83[xe2x80x94O(CH2)mOOC(CH2)nCOxe2x80x94]x
or
[xe2x80x94CO(CH2)mOxe2x80x94]x
and x has a value of at least 3 and preferably x has values such that the polymers have a number-average molecular weight of from about 600 to about 50,000. The term aliphatic polyester also includes the polyesters that would be formed by the condensation reactions between monomers like HOR1OH and HOOCR2COOH in which R1 and R2 are independently chosen from linear or branched hydrocarbon chains containing from 1 to 10 carbons. Also included are aliphatic polyesters that would be formed if more than one type of diol monomer, (i.e. with different R1 groups), and/or more than one type of diacid monomer, (i.e. with different R2 groups), were present in the polymer forming reaction. Also included as aliphatic polyesters are the polyesters that would be formed by the condensation reaction of monomers like HOR3COOH in which R3 is chosen from linear or branched hydrocarbon chains containing from 1 to 10 carbon atoms. Also included are aliphatic polyesters that would be formed if more than one type of acid/alcohol monomer, (i.e. with different R3 groups), were present in the polymer forming reaction. Also included are the aliphatic polyesters that would be formed from the condensation reactions of all three of the above monomer types, if monomers with one type or a variety of different types of R1, R2 and R3 groups were present. The term aliphatic polyester includes these polymers whether or not they are formed from the reaction of monomers like those above. For example, aliphatic polyesters may be formed from cyclic esters like xcex5-caprolactone. Aliphatic polyesters include polymers that are end capped with, for example, hydroxy and/or alkyl groups. Preferably the number-average molecular weight of the aliphatic polyesters is from about 600 to about 50,000.
The term poly(olefin glycol) is meant to include polymers with the formula H(xe2x80x94OR1xe2x80x94)mOH in which R1 is chosen from linear or branched hydrocarbon chains containing from 1 to 10 carbon atoms and m is preferably chosen so as to give a number-average molecular weight from about 600 to about 50,000. Also included are the poly(olefin glycols) that would be formed if more than one type of diol monomer like HOR1OH, (i.e. with different R1 groups), were present in the reaction. The term poly(olefin glycol) includes these polymers whether or not they are formed from diol monomers or, for example, from cyclic ethers like tetrahydrofuran. Poly(olefin glycols) include polymers that are end capped with, for example, hydroxy and/or alkyl groups.
The term poly(olefin carbonate) is meant to include polymers with the formula HO[xe2x80x94R1OCOOxe2x80x94]mR1OH in which R1 is chosen from linear or branched hydrocarbon chains containing from 1 to 10 carbon atoms and m is preferably chosen so as to give a number-average molecular weight from about 600 to about 50,000. Also included are poly(olefin carbonates) that would be formed from the reaction of phosgene with more than one type of diol monomer like HOR1OH, (i.e. with different R1 groups), present in the reaction. The term poly(olefin carbonate) is meant to include these polymers whether or not they are formed from the reaction of diol monomers with phosgene. Poly(olefin carbonates) include polymers that are end capped with, for example, hydroxy and/or alkyl groups.
Preferably the first polymer is chosen from poly(1,6-hexamethylene adipate), poly(1,4-butylene adipate), poly(ethylene adipate), an aliphatic polyester/poly(olefin glycol) copolymer known as Terathane CL-2000, (which is a polycaprolactone-block-polytetrahydrofuran-block-polycaprolactone), poly(ethylene glycol), poly(ethylene glycol) methyl ether, poly(ethylene glycol) dimethyl ether, poly(propylene glycol), polycaprolactone, , poly(hexamethylene carbonate)diol and combinations, mixtures and blends thereof.
Since the material that forms light scattering sites is a polymer it has little or no chance of aggregating into pockets or migrating or diffusing out of the combination the way that a low molecular weight material could if it was used as the dispersed phase in a TRLS material.
The second polymer should provide a matrix for the first polymer. The second polymer or the formation of the second polymer should provide for uniform interspersal of the first polymer within the matrix formed by the second polymer. The second polymer should allow for the formation of light scattering sites when the combination of polymers is cold and should provide for a clear or low light scattering material when the combination of polymers is hot. Preferably, the second polymer provides for miscibility between the liquid state of the first polymer and itself when the temperature of the TS material is above the melting point of the first polymer. The second polymer should serve to maintain the rigidity or xe2x80x9cset upxe2x80x9d nature of the combination of polymers even when the TS material is in its high temperature, clear or low light scattering condition. It is believed to be preferable for the second polymer to have high elasticity and/or a large free volume or large interstitial spaces even at low temperatures like normal room temperatures to allow for light scattering site formation by the first polymer. The term free volume of polymers is used here as it is described in Macromolecules, An Introduction to Polymer Science, especially pp. 348, 349 and 367-369, Edited by F. A. Bovey and F. H. Winslow, Academic Press Inc., (1979). It is preferable that the second polymer have a glass transition temperature below the transition temperature for light scattering site formation. It is preferable that the second polymer have at least some crosslink density so that it may better maintain its solid form even at high temperature in the presence of substantial amounts of the first polymer in its liquid-like form. Preferably the second polymer is chosen from one or more acrylics, one or more copolymers formed from ethylene and acrylic monomers, one or more polyurethanes or combinations thereof. More preferably the second polymer is chosen from UV cure acrylics, poly(methyl methacrylate), poly(ethylene-co-methacrylic acid) and the polymers formed by the reaction of Desmophen 1800, Desmophen 1100, poly(ethylene glycol), poly(propylene glycol), polycaprolactone, poly(hexamethylene adipate), poly(butylene adipate), poly(hexamethylene carbonate)diol, Terathane CL-2000, and combinations thereof with a polyisocyanate chosen from Desmophen N-3200, Desmophen N-100, Mondur MRS-4, Mondur MRS-5 and combinations thereof.
With many useful TS materials the first polymer acts as a completely independent interspersed polymer. For example, when poly(ethylene glycol) dimethyl ether is the first polymer and a crosslinked polyurethane made from a polyester polyol and a polyisocyanate is the second or matrix polymer there is no reaction between the poly(ethylene glycol) dimethyl ether and the polyester polyol or the polyisocyanate even if the poly(ethylene glycol) dimethyl ether is present during the formation of the polyurethane. This is because the terminal hydroxy groups of the poly(ethylene glycol) have been methylated. As another example, when polybutylene adipate, as a first polymer, is present during the formation of a UV cure acrylic matrix there are no expected reaction between the acrylic monomers and the polybutylene adipate during the curing reaction. Also, for example, when polybutylene adipate, as a first polymer, is melt mixed into a matrix formed by, for example poly(ethylene-co-methacrylic acid), there are no expected chemical reaction at all and the polybutylene adipate should be independent of the matrix with respect to attachment to the matrix by covalent bonds.
When a first polymer such as polybutylene adipate, (with a weight-average molecular weight of ca. 12,000), is present during the formation of a polyurethane matrix, some of the end groups on the polybutylene adipate may be hydroxy groups and have some chance of reacting with an isocyanate group. But the equivalent weight of the polybutylene adipate is so large that this reaction is considered inconsequential. Even if one or more of the terminal ends of the polymer chain were xe2x80x9ctied upxe2x80x9d, the chain has so much length that portions of it could still participate in melting when heated and crystallite formation and/or light scattering site formation when cooled. When the second polymer is formed in the presence of the first polymer, the independence of the first polymer decreases when its equivalent weight is of lower values and it contains groups that are reactive with the components used to form the second polymer.
However it has also been discovered that many useful TS materials can be formed from a first polymer and a second polymer in which the second polymer is a crosslinked matrix formed from a portion of the first polymer. For example, a diol endcapped polymer like an aliphatic polyester, a poly(olefin glycol) or a poly(olefin carbonate) can be mixed with a polyfunctional crosslinking material like a polyisocyanate with a less than stoichimetric amount of isocyanate groups in the polyisocyanate as compared to hydroxy groups in the diol endcapped polymer. Only some of the diol endcapped polymer""s hydroxy groups can participate in the formation of the polyurethane matrix. Some of the diol encapped polymer chains will remain free to act as a first polymer and if they fit the criteria for a first polymer, they may provide for TRLS.
Thus, for example, a polymer with hydroxy functionality of at least two can be reacted with a material with an isocyanate functionality of at least three. If a sub-stoichiometric amount of isocyanate relative to the amount of hydroxy groups is used, a second polymer can be formed from a portion of the first polymer and the polyisocyanate and this polyurethane can serve as a matrix for the portion of the first polymer that did not react to form the matrix. As another example, the first polymer can have hydroxy functionality of greater than two and a difunctional or polyfunctional isocyanate can be used at the proper sub-stoichiometric level to react with a portion of the polyol to form a second polymer to serve as a matrix for the remainder of the first polymer that did not react.
In this discovery, if the first polymer is difunctional, its polymer chains can end up in three different reacted forms, i.e. unreacted, reacted at one functional position or reacted at both functional positions. In curing with a sub-stoichiometric amount of an at least trifunctional material, there will be a distribution of the original amount of first polymer between these three possible reacted forms that depends on the relative stoichiometry between the reactive groups. The unreacted chains can serve to give the material the ability to change between light scattering and non-light scattering states. The polymer chains reacted in one position might have a chance to participate in forming light scattering sites to some extent on cooling of the material as the chains have a free end and may partially solidify or crystallize. The chains that are reacted on both end are believed to form a matrix that does not participate in changes in light scattering ability like solidification, crystallization and melting. This is supported by the fact that generally with a true 1 to 1 stoichiometry between reactive groups and with no other polymers present, the cured systems do not exhibit TS character and are either clear and non-light scattering at all temperatures or are hazy and light scattering at all temperatures.
If the liquid form of the first polymer and the solid form of second polymer are miscible, it means that they act like a single phase with little or no light scattering, (see for example Concise Encyclopedia of Polymer Science and Engineering pp 629-632; executive editor J. I. Kroschwitz; published by John Wiley and Sons, Inc. (1990)). One way of picturing the miscible polymer combination in its high temperature, clear state is as polymer matrix containing a high molecular weight plasticizer. Another picture might be that of the first polymer as a high molecular weight solvent which solvates the chains of the second polymer and the second polymer as a matrix which holds the liquid-like first polymer in place. It is likely that this mutual interaction between the polymers which can be referred to as miscibility, gives rise to a lack of light scattering sites at high temperature in the combination of polymers and allows many of the materials of the invention to change to a clear state when heated. When there is miscibility between the liquid form of the first polymer and the matrix formed by the second polymer it means that there is no requirement for index of refraction matching between the dispersed phase and the matrix phase since they are acting as, or actually are, a single phase.
The discovery of first and second polymers that appear to be miscible at temperatures above the melting point of the first polymer and allow for light scattering site formation within the matrix at temperatures below the melting point of the first polymer is significant. In fact, whereas the low temperature or light scattering form might best be considered as a composite of the separate components, when the liquid form of the first polymer is miscible with the second polymer, the high temperature form of the materials of the invention might no longer be considered a composite even though it is a combination of two separate polymer systems.
In general, the materials of the invention change from their light scattering state to their clear state abruptly, (within seconds), on rapid heating to a temperature above the melting point of the first polymer. However, on cooling the change from the clear state to the light scattering state is not nearly as quick. In fact, materials of the invention that have been heated to their clear state can be cooled to temperatures 20C to 30C below their clearing temperature and will typically take 10 to 20 minutes or even as long as 3 to 4 hours, in some cases, to return to their light scattering state. This is presumably due to the slow rate of solidification, (which may involve crystallite formation), of the chains of the first polymer especially when those chains are interspersed and entangled in each other and the second polymer. On the other hand, the steady state temperature at which the clear state can be maintained is only slightly below the temperature at which clearing originally took place. Thus at very low heating and cooling rates there is only a little hysteresis between the clearing and frosting points of the materials of the invention.
TS materials have been discovered that have a transition temperature for changing from light scattering to clear at almost any temperature in the range of about xe2x88x9214C to about 70C. Mixtures of different types or different molecular weights of the preferred polymer useful as first polymers can be used to achieve transition temperatures that are intermediate to single polymer type or molecular weight. Additives also can be effective in modifying the transition temperature of TS materials.
The temperature of transition or range of temperatures over which the transition from light scattering to substantially less light scattering takes place depends on the melting point of the first polymer and the environment of the first polymer provided by the polymer matrix. With the materials of the invention, the first polymer is in such an intimate dispersion with the second polymer that the second polymer can serve as a material that provides melting point depression. If the second polymer provides several different or a range of different environments to the first polymer, portions of the first polymer will melt at different temperatures or over a range of temperatures. In addition as discussed above, in some cases, a portion of the chains of the first polymer have the possibility of being attached to the second polymer on one end and therefore may participate differently in melting and solidification than unattached polymer chains. Thus heating the TS materials of the invention to a temperature above the melting point of the first polymer usually results in a complete transition to the less or in most cases non-light scattering state, but heating the materials to temperatures below the full transition temperature and holding the material at that temperature results in a partial transition or partial clearing. If the temperature is controlled over the proper range of temperatures, the materials can be controlled at intermediate level of light scattering. Therefore, most of the materials of the invention are gray scale controllable with respect to the amount of light scattered by the material over the range from the most light scattering state to the least light scattering state.
The relative amounts of first and second polymer are limited on the end of large amounts of first polymer by the need to have a material that is solid or set up even at high temperatures and at the other end of having small amounts of first polymer by the need to have a material that actually turns light scattering when it is in its low temperature condition. In a few cases for polyurethane based TS materials, where the second polymer was formed from a portion of the first polymer, an attempt was made to use a 1 to 1 ratio of hydroxy groups to isocyanate groups. Unexpectedly, a slight amount of TS activity was observed with these samples, especially when they were quite cold. That this happens may be due the polyisocyanate material having a higher than reported equivalent weight or the material may have lost isocyanate groups due to water absorption from the atmosphere during use or there may be small amounts of residual water in the diol or polyol such that not all of the hydroxy groups from the diol or polyol are tied up in polymer matrix formation. If this is the case some of the diol or polyol polymer chains may be free to participate in light scattering site formation on cooling. However in general, the percentage by weight of the total weight of a TS material made up by the first polymer ranges from about 5% to about 85%. Preferably the weight percentage of the total weight of a TS material made up by the first polymer is from about 10% to about 80%.
In a TS material with an unreactive or slightly reactive first polymer and a polyurethane based second polymer which is formed from the reaction of a hydroxy containing material and a isocyanate containing material, the ratio of hydroxy groups to isocyanate groups can range from 1:2 to 5:1 and is preferably from 1:1 to 4:1.
The extent of the clarity of at least some to the materials of the invention is quite remarkable. Samples have been prepared that appear pure white and highly light scattering in their cool, frosted state and have no observable color, haze or light scattering when viewed with the human eye in there high temperature, clear state even though some samples are at least 4 centimeters thick.
The TS polymer combinations may be formed by making a solution of the first polymer in one of the components that can be reacted to form the second polymer. This solution is usually formed at a temperature above the melting point of the first polymer. One or more additional components that are to be reacted to form the second polymer system are then added along with any desired catalysts and/or initiators. The reaction to form the second polymer is then allowed to proceed. This reaction often takes place at a temperature above the melting point of the first polymer. Volatile by-products of the polymerization reaction, if any, may be removed by heating and/or exposure to vacuum.
In a preferred methods of making the materials of the invention, the first polymer is dissolved in a crosslinkable but as yet uncrosslinked second polymer. This solution is maintained at a temperature above the melting temperature of both of the polymers and a single phase, solution is formed by the two polymers. A crosslinking agent is added. This crosslinking agent may be a difunctional or polyfunctional monomer, oligomer or polymer. The crosslinking reaction may proceed on its own or it may be accelerated by catalytic or initiator means added to the polymer mixture. The crosslinking reaction may be an addition reaction, a condensation reaction, a free radical initiated reaction or any other well know reaction for crosslinking polymer chains. Once the crosslinking reaction has proceeded to a significant extent, the liquid solution will set up to form a non-flowing material. With the TS materials of this type, the set up material will be clear (i.e. little or no light scattering). On cooling the material will change to a substantially greater light scattering state. The preferable second polymers formed by this method are crosslinked polyurethanes.
TS material may be formed by melting the first and second polymer and mixing them together in the melt to form a uniform dispersion or solution. On cooling, the first polymer forms light scattering sites in the matrix provided by the second polymer. To be useful, the melt mixed TS material changes from light scattering to less light scattering at temperatures near the melting point of the first polymer and below the melting point of the second polymer. This allows the second polymer to maintain a solid, although possibly softened, matrix for the first polymer even in the clear state. The preferred second polymers for melt mixing are low molecular weight acrylic polymers and copolymers formed from ethylene and acrylic monomers.
TS materials may be formed by dissolving the first and second polymers in a solvent or solvent system, followed by removal of the solvent(s).
In another preferred method of making materials of the invention, the first polymer is melted and a UV curable material is added. The solution of the first polymer and the UV curable material is held at a temperature above the melting point of the first polymer while the solution is exposed to UV light. The reactions initiated by UV light cause the formation of a second polymer in the form of a matrix surrounding the first polymer. TS site formation takes place on cooling if the UV cure material allows for solidification of the first polymer within its matrix. The preferred UV curable material is an acrylic material.
A TS material may be formed by dissolving one or more monomers which may be reacted to form the first polymer in a solution of a solvent and the second polymer, (or without an added solvent if the second polymer is soluble in the monomer(s) used to form the first polymer). Any desired catalysts and/or initiators are then added and the polymerization reaction to form the first polymer reaction is carried out, normally at an elevated temperature or with exposure to electromagnetic radiation. This is followed by removal of some or all of the solvent if one was used.
A TS combination may be formed by dissolving the first polymer in a solution of monomers, oligimers, crosslinking agents and/or polymer chains that can be reacted to form the second polymer. Optional inert solvent(s), catalyst(s) and/or initiator(s) may be present in the solution. The reaction to form the second polymer is then carried out, followed by solvent removal if present and if removal is desired.
Some of these methods of making TS materials involve having the first polymer in solution with the second polymer or one or more of the components used to form the second polymer. A homogenous solution is the ultimate dispersion or means of interspersing one material in another. When the TS material is formed at temperatures above the melting point of the first polymer, cooling results in light scattering site formation via solidification from the highly dispersed state. Re-heating the TS material allows the first polymer to return to its highly dispersed state to form very clear materials even when the materials are in thick forms. By way of comparison, a TRLS material can be formed by dispersing a fine powder of an aliphatic polyester in the precursors to a polyurethane matrix and then allowing the precursors to react at a temperature below the melting point of the aliphatic polyester. This results in a material that becomes less light scattering when heated but does not become nearly as clear on heating as the TS materials of the invention.
An advantageous method of forming a TS material, for certain applications, is provided by making the TS materials of the invention in the form of powder or particles, which optionally may be beads or spheres, followed by having the powder or particles dispersed in a clear polymer material or layer. The TS material formed in this manner has a high probability of having an optically clear state, with little or no distortion or light scattering, if the clear polymer material in which the particles are dispersed is the same or similar to second polymer that makes up the matrix portion of the TS particles. The TS particles can be formed by making a bulk sample of the TS material by one of the methods described above and then comminuting the material to any desired particle size.
Alternatively, the TS particles can be formed by dispersing the components used to form the particles in an inert liquid, (a liquid in which the components are not substantially soluble and with which the components do not substantially react), followed by allowing the particle forming components to react, if a reaction process, for example crosslinking, is used to form the TS material. The rate of reaction may be facilitated by heating or by the addition of a catalyst or an initiator. The TS particles thus formed may be filtered from the inert liquid and rinsed and dried if necessary.
The particles formed by comminution or by dispersion in an inert liquid or by some other process can be used directly as a TRLS particles in variety of applications, for example, temperature indicating applications. Also, these particles can then be dispersed in a polymer or polymer forming system to form a TS material or layer with the advantage that the polymer or polymer forming system can be formed or processed under conditions that are different from the conditions for the formation of the TS particles themselves. For example, the clear polymer or its precursors in which the particle are dispersed could be dispensed, used in lamination processes, cast or otherwise formed at room temperature while the particles or a TS material that did not involve this particle process would normally be used or processed at temperatures above the melting point of the first polymer.
The following is a general procedure for preparation and use of one type of TS particle. An aliphatic polyester as a first polymer, with a melting point above room temperature, and the precursors to the formation of a polyurethane matrix are stirred together in an inert liquid with an optional surfactant to promote dispersion and/or emulsification. For example, the polyurethane precursors may be a polyester polyol and a polyisocyanate material which are liquids at room temperature. The inert liquid is heated to above the melting point of the aliphatic polyester and the mixture is vigorously stirred to form liquid droplets containing the aliphatic polyester, polyester polyol and the polyisocyanate. In each particle or droplet the polyisocyanate reacts with the polyester polyol, with or without an added catalyst, to form a polyurethane matrix or second polymer in which is interspersed the aliphatic polyester, first polymer. On cooling the light scattering particles are filtered out, rinsed and dried.
These particle can be used as they are as particles that reversibly change between light scattering and clear. Alternatively, they can be dispersed in a clear polymer material to form a TS composite. An advantageous TS composite would be formed by dispersing the particles in the polyester polyol and the polyisocyanate used to make the particles. In this case the particles are surrounded by the same polyurethane that serves as the matrix or second polymer in the particles. In this way the particles, in their clear condition, have a high probability of closely matching the index of refraction of the polymer material in which the particles are dispersed. The aliphatic polyester in this case is localized in the regions made up by the original particle and upon raising the temperature above the melting point of the aliphatic polyester a transition takes place to form clear particles within the clear polymer material surrounding the particles to give a clear or nearly clear overall material.
When a polyurethane is used as the second polymer it can be formed by allowing hydroxy groups to react with isocyanate groups at room temperature or at elevated temperatures. This reaction can be accelerated anywhere from a little to very substantially by the addition of catalysts. The preferred catalysts for accelerating the polyurethane formation reaction are tertiary amines and organotin compounds. Particularly preferred are Desmorapid PP from Bayer Corporation of Pittsburgh Pa. and dibutyltin dilaurate.
The TS polymer materials may be formed by other methods as will be obvious to those skilled in the art of polymer technology.
The TS material can be processed into useful forms by spreading the precursors for the formation of the TS material on a substrate like a sheet of glass or a sheet or film of plastic. The glass or plastic can optionally have been previously or can be subsequently coated with a conductive layer, transparent or non-transparent, to serves as a heater. The precursors can be coated by processes which include drawing, bladeing, roll coating, curtain coating and various types of printing. A variety of types of coating processes useful for the materials of the invention are described in Liquid Film Coating edited by S. F. Kistler and P. M. Schweizer, Chapman and Hall, (1997).
The precursors to the TS materials can also be provided between two substrates such as sheets of glass, sheets of plastic or a combination of the two and the TS material can serve as a lamination layer for the two substrate layers. In the clear state the TS materials can be close in index of refraction to the substrates and suppress reflection from the interface between the TS material and the substrates. Also a free standing sheet of TS material can be used to laminate two substrates together or a free standing sheet of TS material can be laminated in between substrates with other bonding layers like polyvinylbutyral, polystyrene or polyurethane. In certain privacy glass applications it is of significant advantage for the TS layer to bond well to glass and form an effective laminating layer. It has been discovered that some of the TS materials of the present invention are particularly good at forming a laminating layer between sheets of glass. For example, a TS layer made with a 2 to 1 ratio of hydroxy groups to isocyanate groups with PBA-de-1,000 and Desmodur N-3200 forms well bonded layers to glass while some loss of adhesion is observed between the TS layer and sheets of glass at the same ratio of hydroxy to isocyanate with PBA-de-1,000 and Desmodur N-100.
Improved bonding to glass is obtained by incorporating coupling agents in the TS layer precursors when they are used to form a laminating layer with glass substrates. Preferred coupling agents are silane coupling agents and titanate coupling agents. The coupling agents may have groups that react with the precursors or they may be unreactive. The coupling agents may be monomeric or polymeric in nature. An extensive discussion of coupling agents is given in Silane Coupling Agents, 2nd Edition, E. W. Plueddemann, Plenum Press (1991) and Silanes and Other Coupling Agents, K. L. Mittal Editor, VSP BV (1992).
The TS material can be formed into freestanding objects and forms by processes like molding, extrusion and casting. The molding may involve reactive injection molding in which the precursors, including optional catalysts, are meter mixed and then injected into a mold where the precursors react to form the TS material at room or elevated temperatures. A reactive extrusion process can be used to form freestanding sheets of TS material by meter mixing the precursors, including optional catalysts, and then forcing the mixed precursors through a slit into a reaction zone in which the precursors react to form the cured product. Freestanding film materials can be used as is or can be used in subsequent lamination processes to bond substrates together.
An interesting configuration for the TS material is a film of the material with an embedded fine metal mesh to serve as an a resistive heater in intimate thermal contact with the TS layer. The fine metal mesh can be like that used as a resistive heater in heated windshields or windscreens on certain motor vehicles and as such it is highly transparent to light and still low enough in electrical resistance to provide effective heating when electrical power is passed through the mesh. The mesh can be made from a variety of metals and metal alloys including copper, nickel, chromium, silver, steel, stainless steel, nickel-chromium alloys, etc. If this TS film with embedded metal mesh is used in a freestanding configuration, the thermal mass of the system is small compared to a TS layer coated on a fairly large substrate or laminated between two fairly large substrates since in these latter cases the substrate(s) must be heated as well as the TS layer. The freestanding configuration can be a layer or film by itself in air or it can be a part of a double pane or a triple pane glazing unit with gas spaces between the layers of the unit. If it is part of a triple pane glazing unit, it is preferable that the TS layer with embedded metal mesh be the middle pane or layer of the glazing unit. In the double or triple pane glazing units, one or more heat reflective or low emissivity layers can be used, if desired, to minimize heat loss and reduce power consumption for maintaining the TS layer in its high temperature clear or low light scattering mode.
In a multiple pane glazing unit with a TS layer, the layer can also be coated on a sheet of glass or plastic, but if this sheet is relatively thick, it will have relatively large thermal mass and will increase the energy requirement for heating the layer to its clear condition. The advantage of having low thermal mass, that would be achieved with the metal mesh described above, would also be achieved by providing the TS layer on a thin substrate like a polyester film. The polyester film can be coated with a transparent conductor such as the products sold under the names Altair(trademark) or Heat Mirror(trademark) by Southwall Technologies of Palo Alto, Calif. As in other cases, these transparent conductors can serve as heat reflectors and low emissivity layers as well as transparent heaters.
In the case that the TS layer is configured in a relatively low thermal mass situation, it has been discovered that the slow rate of return to the light scattering mode can be used to advantage. The discovery is that the TS layer only needs to be heated to a temperature above its clearing point or clearing temperature for just a brief period of time, for example 5 to 60 seconds and this only needs to be done approximately every 10 minutes to as much as every 3 or 4 hours depending on the TS material involved and ambient temperatures in which the window or device is being used. With this kind of control, the TS layer can be maintained in its clear mode over long periods of time with low to very low overall energy consumption. A timing circuit in conjunction with the power supply can be set to the particular time on/time off requirements for the particular window or device size and configuration and for the particular TS material(s) being used. This control approach is much less effective when the TS layer is on a substrate with relatively large thermal mass as the energy required to intermittently heat the large thermal mass above the clearing point or clearing temperature is nearly as large as the energy consumption for maintenance of the layer and substrate at a temperature that maintains the clear mode.
Another way of controlling and minimizing the power to maintain the clear condition of the TS material in an actively controlled window or device, is to supply power to the means of heating only when the TS layer is starting to return to its light scattering condition. The return to the light scattering condition is readily detected by a light source and a light detector that are near each other and near a portion of or layer of the TS material. The light source may be any type of light source but is preferably a light emitting diode and the light detector may be any type of light detector but is preferably a photodiode or a photoresistor. The light source and the light detector can be aimed directly at each other with the light beam passing through a portion or layer of TS material. In this case, the on-set of light scattering in the material between the source and the detector causes a decrease in the signal output of the light detector. Alternatively the light source can be directed at an off angle with respect to the light detector and the on-set of scattering in the TS material can cause an increase in light directed to or scattered to the light detector and thereby cause an increase in the signal from the detector. In either case, the signal from the light detector can be used in a control circuit to determine when power should be applied to the means of heating the TS layer.
Devices incorporating TS materials whose temperatures are actively controlled can use control mechanisms that include thermostats or thermoregulators in conjunction with electronic control circuits. The temperature can also be regulated by incorporating positive temperature coefficient materials in thermal association with TS layer and in line with the electrical system that provides power to actively control the temperature of the TS material. In this way the positive temperature coefficient materials can prevent over-heating by having their resistance to electric current flow increase with increasing temperature, (especially a dramatic resistance increase starting at a given temperature), and thus decrease the power supplied to the heater in the device. In a similar way a bimetallic switch can be used as a thermoregulator or thermo-shut off mechanism. In general, the control mechanisms can be manual, automatic or involve remote control by infrared or radio frequency signaling. Remote control is especially useful for heater control circuitry in difficult to reach locations or in situation that are difficult to provide with external wiring like with multipane windows.
In addition to the first and second polymer, the TS materials of the invention may contain additives such as other polymers, ultraviolet light absorbers, visible light absorbers, infrared light absorbers, ultraviolet light stabilizers, visible light stabilizers, antioxidants, antiozonants, singlet oxygen quenchers, thermal stabilizers, plasticizers, solvents, dyes and/or pigments.
Good, although not exhaustive lists of antioxidants, singlet oxygen quenchers, light absorbers, light stabilizers and pigments are given in columns 13 and 14 of U.S. Pat. No. 4,425,161 to Shibahashi et. al. and columns 3-7 of U.S. Pat. No. 5,688,592 to Shibahashi et. al. Preferred light absorbers and/or stabilizer materials include benzotriazoles, benzophenones, cyanoacrylates, hindered amines, oxalanilides and substituted triazines. Materials that are not good UV absorbers but provide increased stabilization in the TS material are hindered amine light stabilizers, (HALS). Preferred light absorbers and light stabilizers for use in the TS material of the invention are those described by M. Dexter in xe2x80x9cKirk Othmer Encyclopedia of Chemical Technology, 3rd Edition, Vol. 23, Pp. 615-627, John Wiley and Sons, Inc. (1983). Most preferred are 2-hydroxy-4-(octyloxy)benzophenone from Aldrich Chemical Company Inc. of Milwaukee, Wis.; Uvinul 3039, (2-ethylhexyl 2-cyano-3,3-diphenylacrylate) available from BASF Corporation of Rensselaer, N.Y.; Norbloc 6000, (2-(2xe2x80x2-hydroxy-5xe2x80x2-(2-hydroxyethyl)phenyl)benzotriazole) available from Janssen Pharmaceutica of Titusville, N.J.; Tinuvin 144, (bis(1,2,2,6,6-pentamethyl-4-piperidinyl) (3,5-di-tertbutyl-4-hydroxybenzyl)butylpropanedioate), available from Ciba Specialty Chemicals of Tarrytown, N.Y. and Tinuvin 213, (poly(oxy1,2-ethanediyl)-xcex1-[3-[3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxyphenyl]-1-oxypropyl]-xcfx89-[3-[3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxyphenyl]-1-oxypropyl] and poly(oxyl ,2-ethanediyl)-xcex1-[3-[3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxyphenyl]-1-oxypropyl]-xcfx89-hydroxy), available from Ciba Specialty Chemicals of Tarrytown, N.Y.
Preferred visible light absorbing dyes are Quinoline Yellow, (Solvent Yellow 33, C.I. 47000); Celestine Blue, (Mordant Blue 14, C.I. 51050); Quinizarin, (1,4-dihydroxyanthraquinone, C.I. 58050) and Malachite Green Carbinol base, (Solvent Green 1, C.I. 42000:1) all available from Aldrich Chemical Company Inc. of Milwaukee, Wis. Preferred NIR absorbing dyes are Keysorb 970 and Keysorb 1026 available from Keystone Aniline Corporation of Chicago, Ill.
Additives, if present, are typically added at a levels from where they first become effective in providing their intended effect, (such as stabilization, tinting or modification of clearing temperature or speed), up to level where they reach their solubility limit in the TS material system or a level at which they interfere with the function or physical properties of the TS material. Other guidance on the level of additives can be obtained from the examples given below and the general practice in the art of polymer additives.
A type of additive, other than those listed above, has been found to have a beneficial effect on the performance of the TS materials of the invention for certain applications. In particular, the addition of fumed silica has been found to retard or enhance the rate of return to the light scattering condition upon cooling while having very little effect on the clarity of the high temperature, xe2x80x9cclearxe2x80x9d state. A wide variety of fumed silicas and treated fumed silicas are of interest including Cab-O-Sil(copyright) products from Cabot Corporation of Naperville, Ill. and Aerosil(copyright) products from Degussa A. G. of Frankfurt, Germany.
We have found a number of uses, some of which are believed to be unique, for the TS layers of the present invention and for TRLS materials in general. For example, a TRLS layer may be used in a situation where privacy is desired at some times and a clear view is desired at other times and in some situations these windows eliminate the need for costly window treatments like shades and blinds. Windows that incorporate TRLS materials are useful in conference rooms, offices, as part of office partitions, windows on jewelry store display cabinets, windows on cabinets that contain valuable items like stereos, televisions and safes, windows that separate stores from public areas in shopping malls, and windows in office building and houses. Another privacy glass application involves shower doors. An application that automatically provides cooling to speed transition to the frosted state involves variable transmission windows on refrigerators, freezers and display coolers in stores. The TRLS layer may cover part or all of the window in all of the above window applications and may be applied as a uniform layer or in certain decorative patterns like stripes, circles or diamonds. Several different TRLS materials, each with a different temperature for clearing/frosting, can be used and the temperature of the layer can be controlled to clear/frost some or all of these TS materials when clearing/frosting is desired. The layer of TRLS materials may vary in thickness across a window or device and clear in a sweeping or a shutter type manner. The window applications that involve sunlight exposure may incorporate enough sunlight energy absorbing character, in thermal association with the TS material, such that the sun provides all or a portion of the energy to cause the window to spontaneously clear in sunlight.
TRLS materials are useful in variable reflectance mirrors. A light scattering layer formed on or near a specularly reflecting layer still appears highly light scattering when it is in its scattering condition and provides for good specular reflection when the material is in its clear condition. For specularly reflecting layer in displays, localized areas with TRLS material provide excellent contrast with those areas not coated with TRLS when the materials are in their scattering condition and provide little or no contrast when the TRLS materials are in their clear condition. This is especially true with the TRLS materials of the present invention. The lowest contrast for this and other display applications is obtained when the areas that are not covered with TRLS material are covered with a material that matches the characteristics of the TRLS material in the clear condition. For the TS materials this is mostly readily achieved by surrounding the TS areas with areas of the second polymer that do not contain the first polymer are hence are very close in nature to the TS material.
TRLS materials are useful as a temperature indicators. Applications using this feature include a TRLS coating on part or all of a baby bottle that would clear when the contents were warm or hot and would turn light scattering when the contents were cooled. Another application is a warming bag for, for example, food or pharmaceuticals which bag indicates when the contents has reached a certain temperature by changing from frosted to clear and a freezer/refrigerator bag that with the proper choice TRLS material turns from clear to frosted at freezing or some other desired temperature and turns back from frosted to clear on thawing or warming. A desirable use of the TRLS materials of the invention involves shower heads which change from frosted to clear to indicate when the water passing through and out of the shower head is warm or hot. Similarly the TRLS materials are useful as temperature indicators in a variety of water faucet or water line application in which there is a desire to know if water flowing through the faucet or line has reached a particular temperature, either hot or cold. Other temperature indicator applications include hot water bottles, tea cups, coffee cups, beer mugs, coolers and vacuum insulated bottles and flasks. In all of the temperature indicating applications above, the items or products may have areas, bands, stripes or spots provided by several different TRLS materials with different temperatures for changing between frosted and clear or TRLS materials of different thickness or controlled variation in thickness. This will allow the materials to indicate just how hot or how cold are the product or the contents of the product which incorporates the TRLS materials on the basis of how many stripes, bands or spots are clear or what proportion of a variable thickness layer clears in a certain time. Also the TRLS layers may reversibly reveal and conceal symbols and/or numbers based on the temperature of the materials.
Other applications include placing the TRLS materials or layers in association with things that become warm or hot in there normal use. One example of this type application is provided by the doors of a fireplace or wood burning stove that clear up when the fireplace or stove is in use but frost up and obscure the view of the inside when the fireplace or stove is not in use. Another application involves see through doors or window packs in doors or window packs in other places on conventional, convection, microwave and hybrid ovens that change from frosted to clear while the oven is in use to reveal the contents of the oven and change from clear to frosted to hide the contents of the oven when it is not in use. TRLS layers can be used to provide variable transmission for a window or a portion of the cover for gas or charcoal grills. In these window applications, the see through doors and windows optionally involve multipane construction with gas spaces between the panes and optionally involve heat reflective coatings on the transparent layer(s).
TRLS layers or coatings are also useful in conjunction with light and lighting fixtures, including automobile head lamps, such that the layers heat up and clear when the lights are turned on and frost up and obscure the inside of the light or fixture when the lights are off. Among other places, this is of interest for high intensity discharge lamps in motor vehicles where the lamp housing is not sealed and unsightly condensation causes appearance concerns. Other applications involve providing variable transmission possibilities to appliances like bread makers, pizza makers, slow cookers, rotisseries, rice makers, toasters and toaster ovens, shaving lotion dispensers and heating pads.
In conjunction with microwave ovens and other microwave sources, it has been discovered that the TS materials of the invention are effective absorbers of the wavelength of microwaves that are used in household cooking type microwave ovens. Thus microwaves are an effective means of heating the material of the invention and thus an effective means of causing the materials to change from light scattering to clear. If a microwave beam is directed in a localized area it is effective to cause localized clearing. Since the materials are effective absorbers of microwave energy they are useful as barriers to the passage or leakage of microwaves and are effective in suppressing reflection of microwaves of certain wavelength from objects coated with the materials of the invention. The TS materials can be incorporated in various containers for use in microwave ovens to provide localized heating and/or indication of temperature of a container or its contents.
An application that also involves spontaneous heating under undesirable conditions involves fires in homes and buildings. TRLS layers can be provided in association with fire rated glass to provide indication of fire and serve to indicate where there are hot spots by changing from frosted to clear. TRLS layers on fire doors or even on walls can turn clear under fire conditions to reveal warnings underneath the TRLS layers.
TRLS materials that clear on heating are useful in signs or displays in at least three configurations. One configuration involves a layer of TRLS material which uniformly covers the information such as letters, numbers and symbols. When the layer is cold the information is obscured and when the layer is warm or hot and layer is clear, the information is revealed. A second configuration involves a layer which contains both the desired information and a background that match each other when the TRLS material is in its light scattering condition. Either the desired information or the background is made up of the TRLS material and the other is made up of a thermally inactive material. The information is revealed when the layer is heated above the clearing point as either the information or the background clears and reveals the information in contrast to the other part of the layer.
The third configuration involves a layer which contains both the desired information and a background that match each other when the TRLS material is in its clear condition. Either the desired information or the background is made up of the TRLS material and the other is made up of a thermally inactive material. The information is revealed when the layer is cooled below the frosting point as either the information or the background turns light scattering and reveals the information in contrast to the other part of the layer.
The displays can spontaneously change due to changes in the temperature of the environment around the displays such as with warning signs that would indicate, for example, the possibility of freezing or slippery conditions or the fact that something is hot like beach sand on a sunny day. Alternately, the temperature of the sign or display can be actively controlled by heating the TRLS material by a variety of means including a resistive heater supplied with electric power. Cooling of actively controlled signs or displays can be provided by allowing heat to be dissipated to the environment. Also, relatively high speed, reversible appearance and disappearance of information is provided by placing the TS material based display or sign in association with a thermoelectric device that can alternately provide either heating or cooling or by the use of a conventional heater and another active cooling system like a fan, a flowing liquid or a refrigeration unit. The actively controlled displays are useful for point of purchase displays and almost any type of information display in which slow switching is acceptable, including relatively slow multi-segment displays with relatively high information density.
The preferred TRLS materials in all of the application described are the TS materials of the present invention. TS materials are useful in providing variable transmission light scattering control to portions of almost any device or process that involves temperature changes.
Maintaining the TS materials at high temperatures while they are exposed to the atmosphere for prolonged periods of time, (e.g. maintaining TS window at 85C continuously for 2 months), can cause the TS material to turn yellow at the edge of the window. This is presumably due to oxygen and/or moisture effects on the material at this temperature. It has been observed that the unexposed portion of the TS material are unaffected by prolonged high temperature operation. This sensitivity to exposure to the environment at elevated temperatures can be decreased by the use of additives like antioxidants and thermal stabilizers. Another effective means of stabilizing the TS materials for prolonged high temperature operation involves encapsulating the materials or edge sealing devices that contain the materials. Encapsulants and edge sealants can be low melting glasses, thermoplastic materials or thermoset materials. Preferred edge sealants are epoxies with low oxygen and moisture permeability.
A TS window can be prepared by having free standing layer of TS material or by coating a layer of TS material on a substrate like a sheet of glass. If the substrate is plastic it may be rigid or flexible and may be acrylic, (e.g. polymethylmethacrylate), sheet material, polycarbonate sheet material, polyester, various types of vinyl, fluorocarbon polymers, polyolefins, polystyrene, polyurethane, acetate or any plastic material that can be formed into transparent sheets, either rigid or flexible. A preferred plastic substrate is a polyester sheet or film.
If the substrate is glass it may be soda/lime glass, borosilicate glass or any of a variety of clear or tinted glass types commonly known in the art of glass making. The glass may be formed into sheets by various processes including the drawn sheet process or the floatline process. Preferred is soda/lime glass, particularly soda/lime glass made into sheet form in the floatline process.
The TS layers on the substrate can be from about 0.05 millimeters to about 4 millimeters thick. The layers can be uniform or vary in thickness. Several different TS materials can be coated on a single substrate either in different areas or different layers which overlay each other.
A major concern for actively controlled TS windows of the invention is the energy requirement for heating to a temperature to cause switching from the light scattering mode to the clear mode and the energy requirement for maintaining the window in the clear mode. While having the TS material on a substrate with low thermal mass decreases the energy requirement for initial clearing, the rate of heat loss or dissipation to the surroundings, is often similar for windows with substrates of high and of low thermal mass. An effective method for decreasing the rate of heat loss is to provide a confined gas space on one or both side of the TS window pane. The confined gas spaces are provided by separate substrate(s) in a parallel spaced apart relationship to the substrate with the TS layer and by an edge spacer/seal like that in double or triple pane windows, also known as insulated glass units, (IG units).
The space between the substrates in a multipane TS window configuration can be evacuated or it can be filled with a gas like air, nitrogen, carbon dioxide, sulfur hexafluoride, argon, xenon or krypton. The thickness of the gas space will have an optimum value based on the thermal conductivity of the gas chosen and the requirements for minimization of thermally induced convection. Apart from the excellent insulating properties of a vacuum, a gas like krypton with its low thermal conductivity and low kinematic viscosity is preferred since its optimum spacing is relatively thin compared to other gases and having a thin gas space helps minimize the effects of thermal expansion in the gas space due to heating of the gas. Inert gasses can also serve to protect the TS layer from the effects of high temperature exposure to oxygen and moisture.
In addition to low heat transfer gasses, the heat loss from the TS window structure can be decreased further by the use of heat reflective and low emissivity coatings. A preferred placement of these coating is such that they effectively reflect heat or infrared radiation back to the TS layer or minimize radiative type thermal loss from the TS layer. As the heat reflective and low emissivity layers are relatively expensive there number should be minimized and there placement should be judicious.
On the other hand, the heat loss from an actively heated window can be used to advantage in certain window situations where interior heating is in use and where having a window with a warm interior surface minimizes thermal convection of inside air past an otherwise cold window surface. This window configuration provides the opportunity for privacy when the window is not heated and provides the opportunity to contribute to heating and minimization of heat loss when it is heated.
A preferred multipane TS window configuration involves a first substrate, as an outside pane, which is coated with a heat reflective layer on the side facing the interior of the configuration. A second substrate, as a middle pane, in a parallel, spaced apart relationship to the first substrate which second substrate is coated on the side facing the first substrate with a TS layer and on the side facing away from the first substrate with a low emissivity layer. Optionally this low emissivity layer, functions also as a transparent heater if active control of the temperature of the TS layer is desired. This preferred configuration includes a third substrate, as another outside pane, in a parallel, spaced apart relationship to the second substrate which third substrate is generally not coated with a heat reflective or low emissivity layer. The substrates may be glass or plastic. This window configuration can be installed in a building structure with either the first or the third substrate exposed to or facing the exterior of the building and could be used to provide a clear view at some times and privacy or obscured view at other times.
In addition to the above preferred configuration many other multipane window configuration are possible. These vary with regard to number of substrates, placement of the TS layer(s), placement of the low emissivity or heat reflective layer(s), the spacing between the substrates, the type gas or lack thereof between the substrates.
The transparent heater layers, heat reflective layers and low emissivity layers may be used in windows and other devices of the invention. The transparent heater layers, heat reflective layers and low emissivity layers may be thin metal layers, transparent conducting metal oxide layers, multilayer stacks of metal oxide layers to enhance optical properties or multilayer stacks of alternating layers of metals and metal oxides to enhance optical properties. Preferred heat reflective and low emissivity layer on various substrates are TEC(trademark)15 and Energy Advantage(copyright) Low-E available from Pilkington-Libbey Owens Ford of Toledo, Ohio; LoË-178 and LoE2 available from Cardinal Glass of Spring Green, Wis.; Comfort E and Comfort E2 available from AFG Industries, Inc. of Kingsport, Tenn.; Sungate(copyright)100, Sungate(copyright)500 and Sungate(copyright)1000 available from PPG Industries, Inc. of Pittsburgh, Pa. and Altair(trademark) and Heat Mirror(trademark) layers on polyester films available from Southwall Technologies of Palo Alto, Calif.